1. Field of the Invention
The present invention relates generally to hydraulic down hole or down-the-hole drills.
The output power of down-the-hole drills and the penetration rate are usually low as compared with top-hammer drills. Still, such down hole drills are widely used because the overall cost per meter of drilling distance or depth is comparatively low for this kind of drilling and because the penetration rate is not substantially reduced as the hole becomes deeper.
Conventionally, the drive fluid is compressed air, but hydraulically operated down-the-hole drills are known at least in the patent literature. In practice, however, severe difficulties have been encountered when a hydraulic drive fluid is also used as a flushing fluid. Top hammer drills generally use a closed hydraulic circuit so that they can be operated by oil.
Most down-the-hole rock drills are generally pneumatic. In air operated hammers using open systems are usual because of the somewhat small outer diameter of a down-the-hole rock drill. The output of impact energy is hence rather small, as compared to a modern top hammer. An open hydraulic system for a down-the-hole hammer cannot usually use oil. Further, the lack of lubricant will often be a severe problem.
It has thus been proven difficult to combine high impact power, reliable operation and expected long life of drill components.
Accordingly, it is an object of the invention to provide a down-the-hole drill which combines high output impact power, power economy, and long expected life of drill components.